Growth of carbon nanotubes using metal-free nanoparticles

ABSTRACT

The present invention provides a method for forming at least one carbon nanotube ( 16 ) by using metal-free catalyst nanoparticles ( 14 ), for example Si or Ge comprising nanoparticles. The method uses the step of decomposing a carbon source gas to form carbon fragments which then recombine at the metal-free catalyst nanoparticles ( 14 ) to grow carbon nanotubes ( 16 ). The method according to embodiments of the invention leads to carbon nanotubes ( 16 ) which do not comprise metal impurities.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the growth of carbon nanotubes. Moreparticularly, the present invention relates to the growth of carbonnanotubes using metal-free nanoparticles.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) in general exhibit very good electronic andmechanical properties. Therefore, CNTs are expected to find a largediversity of industrial applications. One of these applications could bethe use as both passive and active components in nano-electronicdevices.

The most commonly accepted growth mechanism for CNTs is based oncatalytic decomposition of a carbon source on a surface of a metalnanoparticle which acts as catalyst in the CNT synthesis. According tothis growth mechanism, the hydrocarbon source decomposes onfront-exposed surfaces of the metal nanoparticle thereby releasinghydrogen and carbon, which dissolves in the nanoparticle. The dissolvedcarbon then diffuses through the metal nanoparticle and is precipitatedto initiate formation of CNTs.

One of the key issues in the growth mechanisms described in the priorart is the need for a metal catalyst particle to initiate the carbonnanotube growth. A disadvantage thereof is that the metal catalystparticles can lead to the presence of impurities in the grown CNTs.Before the CNTs can be used in many applications, these impurities haveto be removed. A variety of chemical and thermal oxidative treatmentsare usually required to remove the unwanted metal impurities from theCNTs. For example, a multi-step purification procedure may be used whichinvolves the use of nitric acid reflux and thermal oxidation.

Catalyst-free growth of CNTs has been achieved previously by using laserablation and arc discharge CNT growth. However, these methods requirevery high temperatures, i.e. temperatures of above 3000° C. Due to thesehigh required temperatures, these methods are not suitable for in-situCNT growth and consequently require an ex-situ approach. Furthermore,these methods may give low production yields compared to CVD methodsthat can be performed at relatively low temperatures (450-1100° C.), canbe in-situ or ex-situ, and give mass production yields.

In Nanoletters, 2002 Vol. 2, No. 10, 1043-1046 (Derycke et al.),catalyst-free CNT growth has been reported to occur on SiC(111) above1500° C. The catalyst-free growth of CNTs is in this document achievedby repetitive annealing a carbon face of hexagonal silicon carbide invacuum at predefined temperature ranges. The CNTs are produced withoutthe use of a metal catalyst but these CNTs grow with their axis parallelto the surface, or in other words aligned to the substrate, and cannotbe considered feasible for mass production of CNTs.

In Applied Surface Science 245 (2005) 21-25 (Wang et al.) carbonnanotips are grown on a silicon substrate without the use of a catalystby using plasma-enhanced hot filament chemical vapor deposition using amixture of methane, ammonia and hydrogen as reaction gas. The carbonnanotips formation is realized by first growing a carbon film on thesilicon substrate during a time period of an hour. A combination offurther growth of the carbon film and ion bombardment by applying anegative bias of 430 V to the silicon substrate produces glow dischargeand makes growth of the carbon nanotips possible.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide agood method for growing carbon nanotubes on a substrate.

The above objective is accomplished by a method according to the presentinvention.

It is an advantage of a method according to embodiments of the inventionthat the carbon nanotubes grown by this method do substantially notcomprise metal impurities.

A method according to embodiments of the invention may use ChemicalVapour Deposition.

The present invention provides a method for forming at least one carbonnanotube. The method comprises:

providing at least one metal-free catalyst nanoparticle to a ChemicalVapor Deposition reactor,

forming reactive carbon fragments by decomposing a carbon source gas inthe Chemical Vapor Deposition reactor, and

recombining the reactive carbon fragments on top of the at least onemetal-free catalyst nanoparticle to grow the at least one carbonnanotube.

A method according to embodiments of the present invention leads toformation of carbon nanotubes which do not comprise metal impurities.

During decomposing the carbon source gas and growing the at least onecarbon nanotube, the temperature of the substrate may be kept between800° C. and 1000° C.

According to embodiments of the invention, providing at least onemetal-free catalyst nanoparticle to the Chemical Vapor Depositionreactor may be performed by:

providing at least one metal-free catalyst nanoparticle on a substrate,and

transferring the substrate with the at least one metal-free nanoparticleon it to the Chemical Vapor Deposition reactor.

According to these embodiments, the at least one carbon nanotube can beformed on a substrate.

According to embodiments of the invention, decomposing the carbon sourcegas may be performed by using a hot filament, by using a plasma, or byusing a combination of a hot filament and a plasma.

The hot filament may be a metallic filament such as a W filament or a Tafilament. The hot filament may have a temperature suitable fordecomposing or cracking the carbon source gas. For example when a hotfilament is used for decomposing the carbon source gas, the filament maybe kept at a temperature of 950° C.

According to embodiments of the invention, providing at least onemetal-free catalyst nanoparticle to a Chemical Vapor Deposition reactormay be performed by providing at least one semiconductor comprisingnanoparticle, e.g. a Si or Ge comprising nanoparticle.

The at least one Si comprising nanoparticle may, for example, be a SiC,a SiO₂ or a pure silicon nanoparticle.

The at least one Ge comprising nanoparticle may, for example, be a GeO₂or a pure Ge nanoparticle.

According to embodiments of the invention, providing at least onemetal-free catalyst nanoparticle on a substrate is performed by:

providing a thin layer of metal-free catalyst material, e.g. asemiconductor material catalyst material, onto the substrate, and

annealing the thin layer of metal-free material so as to break it up andform the at least one metal-free catalyst nanoparticle.

Annealing may be performed at temperatures of between 500° C. and 800°C.

According to embodiments of the invention, the at least one metal-freecatalyst nanoparticle may have a diameter of between 0.4 nm and 100 nmor of between 0.4 nm and 50 nm.

According to embodiments of the invention, the method may furthermorecomprise, before providing at least one metal-free catalyst nanoparticleon the substrate, providing a barrier layer on the substrate forpreventing interaction, e.g. chemical interaction, of the at least onemetal-free catalyst nanoparticle with the substrate.

According to further embodiments of the invention, the method mayfurthermore comprise pre-treating the at least one metal-free catalystnanoparticle before providing it to the Chemical Vapor Depositionreactor. An example of such a pre-treatment may be removal of a nativeoxide (e.g. SiO₂ in case of Si nanoparticles) by means of, for example aHF dip (e.g. a dip in 2% HF for 5 minutes).

According to embodiments of the invention, the carbon source gas may bea hydrocarbon gas having one (C1) up to three (C3) carbon atoms. Thecarbon source gas may, for example, be CH₄, C₂H₄, C₂H₂ or C₃H₆.

According to other embodiments of the invention, the carbon source gasmay be CO.

The CVD reactor in which the method according to embodiments of theinvention is performed may comprise an inert gas and hydrogen. The inertgas may, for example, be nitrogen.

The flow of gasses in the CVD reactor may, for example, be 4 l/min N₂, 2l/min H₂, 0.5 l/min C₂H₂ or 0.1 l/min C₂H₂.

In a further aspect, the present invention provides a carbon nanotubegrown from a metal-free catalyst nanoparticle. It is an advantage thatthese nanotubes are free from metal impurities.

In yet a further aspect, the present invention provides the use of ametal-free catalyst nanoparticle to grow a carbon nanotube. It is anadvantage that these nanotubes are free from metal impurities.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

Although there has been constant improvement, change and evolution ofdevices in this field, the present concepts are believed to representsubstantial new and novel improvements, including departures from priorpractices, resulting in the provision of more efficient, stable andreliable devices of this nature.

The above and other characteristics, features and advantages of thepresent invention will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thisdescription is given for the sake of example only, without limiting thescope of the invention. The reference figures quoted below refer to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

All drawings are intended to illustrate some aspects and embodiments ofthe present invention. Not all alternatives and options are shown andtherefore the invention is not limited to the content of the givendrawings. Like numerals are employed to reference like parts in thedifferent figures.

FIG. 1 illustrates a method for forming metal-free CNT onto Si particlesaccording to embodiments of the present invention.

FIG. 2 and FIG. 3 schematically illustrate a reactor which can be usedfor growing metal-free CNTs on a substrate according to embodiments ofthe present invention.

FIG. 4, FIG. 5 and FIG. 6 illustrate a Scanning Electron Microscopypicture after growth of CNTs onto Si nanoparticles according toembodiments of the present invention.

In the different figures, the same reference signs refer to the same oranalogous elements.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

The invention will now be described by a detailed description of severalembodiments of the invention. It is clear that other embodiments of theinvention can be configured according to the knowledge of personsskilled in the art without departing from the true spirit or technicalteaching of the invention, the invention being limited only by the termsof the appended claims.

The present invention provides a method for forming at least one carbonnanotube (CNT). The method comprises:

providing at least one metal-free catalyst nanoparticle to a ChemicalVapor Deposition (CVD) reactor,

forming reactive carbon fragments by decomposing a carbon source gas inthe Chemical Vapor Deposition reactor, and

recombining the reactive carbon fragments on top of the at least onemetal-free catalyst nanoparticle to grow the at least one carbonnanotube.

According to embodiments of the invention, the at least one carbonnanotube may be formed on a substrate. According to these embodimentsthe at least one metal-free catalyst nanoparticle may be provided on asubstrate and the substrate with the at least one metal-free catalyst onit may then be transferred to the CVD reactor for the growth of CNTs.

The CVD method used for growing CNTs may be thermal CVD or Plasmaenhanced CVD (PE-CVD).

The method according to embodiments of the invention can be applied forgrowing CNTs according to a “base growth” principle or a “tip growth”principle. Occurring of a particular kind of growth principle depends oninteractions between the catalyst nanoparticle and the underlyingsubstrate. The term “base growth”, also referred to as “rooth growth”refers to a growth mechanism where the nanoparticles used to initiatethe CNT growth stay located at the substrate during growth. The term“tip growth”, also referred to as “top down growth” refers to a growthmechanism where the CNTs growth having the CNT situated at the surfaceduring growth and the catalyst nanoparticle on top of the CNT.

Furthermore, the term “non-metal containing” nanoparticles refers tonanoparticles comprising a material different from a metal and suitableto be used as a catalyst nanoparticle for initiating the growth of CNTs.According to embodiments of the invention, any non-metal containingnanoparticles can be used. According to embodiments of the invention,the nanoparticles may comprise a semiconductor material such as siliconor germanium. For example, the nanoparticles may be silicon comprisingnanoparticles and may, for example, comprise pure Si, SiO₂ or SiC or maybe germanium comprising nanoparticles and may, for example, comprisepure Ge or GeO₂. According to specific embodiments of the invention, thenanoparticles may be pure Si nanoparticles or pure Ge nanoparticles.Whenever in the description of the present invention reference is madeto catalyst nanoparticles it has to be understood that non-metalcontaining catalyst nanoparticles are meant.

A method according to embodiments of the invention allows synthesis ofCNTs which do not comprise metal impurities because the growth startsfrom suitable non-metal containing catalyst nanoparticles onto which theCNT growth according to embodiments of the invention can take place.Hence, no purification process is required after formation of the CNTs.

Furthermore, a method according to embodiments of the invention issuitable to be used for massive CNT growth and can be used in highproduction yield applications.

In general, the size of the catalyst nanoparticles may have an impact onthe final diameter of the CNTs formed or, in other words, may determinethe final diameter of the CNTs. The catalyst nanoparticles suitable tobe used for growing CNTs according to a method of embodiments of thepresent invention may have a diameter in the range of between 0.4 nm and100 nm or between 0.4 nm and 50 nm.

Hereinafter, a method for growing CNTs will be described by means ofFIG. 1. It has to be understood that the sequence of steps describedhereinafter is not intended to limit the invention in any way.

In a first step, a substrate 10 is provided (see FIG. 1). In embodimentsof the present invention, the term “substrate” may include anyunderlying material or materials that may be used, or upon which CNTsmay be grown. According to embodiments, the term “substrate” may includea semiconductor substrate such as e.g. a doped or undoped silicon,gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), indiumphosphide (InP), germanium (Ge), or silicon germanium (SiGe) substrate.The “substrate” may include, for example, an insulating layer such as aSiO₂ or an Si₃N₄ layer in addition to a semiconductor substrate portion.Thus the term “substrate” also includes glass, plastic, ceramic,silicon-on-glass, silicon-on-sapphire substrates. The term “substrate”is thus used to define generally the elements for layers that underlie alayer or portions of interest, in particular for the present inventionthe CNTs to be grown. According to a specific embodiment of the presentinvention, the substrate 10 may be a semicondcutor wafer, e.g. a Siwafer or a Ge wafer. According to embodiments of the invention, a majorsurface of the substrate 10 should be inert with respect to CNT growthor should be such that it does not interact with the catalystnanoparticles formed on it. Therefore, according to embodiments of theinvention a barrier layer 11 may be provided onto the substrate beforecatalyst nanoparticles are formed on it (see further).

A thin layer 12 of non-metal material, also referred to as metal-freematerial, is provided, e.g. deposited onto a major surface of thesubstrate 10. This layer 12 may, for example, comprise seumiconductormaterial such as Si or Ge. In case of, for example, Si comprisingmaterial, this thin layer 12 may be a uniformly deposited thin layer,such as a poly-Si (polycrystalline Silicon), amorphous silicon orsilicon dioxide layer deposited by commonly used deposition techniquessuch as, for example, CVD (Chemical Vapor Deposition). The thickness ofthe thin layer 12 may be less than 15 nm and may, for example, bebetween 0.4 nm and 5 nm. According to embodiments of the invention, thethin layer 12 may also be a non-uniform sub-atomic layer deposited bye.g. ALD (Atomic Layer Deposition). Alternatively spin-on and dipcoating techniques may be used to deposit a uniform thin layer 12.

If needed, a barrier layer 11 can be deposited onto the substrate 10before the deposition of the thin layer 12 (see FIG. 1). The barrierlayer 11 may, for example, be used to prevent reaction of the materialof the thin layer 12, e.g. semiconductor layer such as Si or Ge layer,and/or formed nanoparticles with the substrate 10 underneath. Thebarrier layer 11 may, for example, be a Si₃N₄ layer or any othersuitable layer that prevents reaction of the material of the thin layer12 with the substrate 10.

After deposition of the thin layer 12, an annealing step may beperformed to break up the thin layer 12 and to form nanoparticles 14(see step 13 in FIG. 1). The formed nanoparticles 14 may have a diameterof between 0.4 nm and 100 nm and may, for example, have a diameter ofbetween 0.4 nm and 50 nm. FIG. 1 illustrates the formation of thenanoparticles 14. To control the size, more particularly to control thediameter of the nanoparticles 14, the thickness of the deposited thinlayer 12 as well as the temperature and time of the annealing step maybe controlled or well-chosen. The optimal temperature and time to createthe nanoparticles 14 depends on the type and the thickness of thedeposited thin layer 12 of metal-free material. For example, thetemperature for annealing may range between 500° C. and 800° C. Theanneal step may be performed in a reactor. In the reactor, gases such asnitrogen and/or hydrogen can be used as ambient gases.

According to an alternative embodiment, nanoparticles 14 which may, forexample, comprise pure semiconductor material, e.g. pure Si, may beformed in a thin dielectric layer, e.g. a SiO₂ layer, which is provided,e.g. deposited for example by CVD, onto the substrate 10, for exampleonto a semiconductor wafer, e.g. Si wafer. After deposition of the thinSiO₂ layer a low energy Si ion implantation step may be performed on theSiO₂ layer followed by an annealing step to create Si nanocrystals. Adissolving treatment, e.g. HF treatment (e.g. HF vapor or dilutesolution), can then be applied to remove the SiO₂ such that Sinanoparticles 14 which are suitable for use as initiators of CNT growthare left on the substrate 10.

According to still other embodiments of the invention, the substrate 10onto which the nanoparticles 14 are formed or deposited, may be formedof a porous material. Examples of suitable porous materials to be usedwith embodiments of the present invention may be zeolites and porouslow-k materials (commonly used in semiconductor processing andcommercially available). Using porous material or in other words using asubstrate 10 having inner pores makes it possible to deposit the thinlayer 12 not only on the major surface of the substrate 10 but alsowithin these inner pores of the substrate 10. This significantlyincreases the surface area onto which nanoparticles 14 can be formed. Asa result the amount of CNTs which can be formed by the method accordingto embodiments of the invention may also significantly increase.

In case of such porous substrates 10, a thin layer 12 of, for example,semiconductor material, e.g. Si, the layer being continuous or noncontinuous, may be deposited onto a major surface of the poroussubstrate 10 and on the surface of the inner pores of the poroussubstrate 10. After performing of an annealing step as described aboveto create nanoparticles, nanoparticles 14 may be formed on the majorsurface of the substrate 10 and in the inner pores of the substrate 10.These nanoparticles 14 can then be used as catalysts to grow CNTs.

According to yet another alternative embodiment, no substrate is usedbut bulk catalyst nanoparticles may be provided to grow CNTs. The bulknanoparticles should be such that a carbon source gas is able to flow inbetween neighbouring nanoparticles such that, when the bulk catalystnanoparticles are provided in a reactor, CNTs can be grown onto thecatalyst nanoparticles (see further).

According to embodiments of the invention, the nanoparticles 14, e.g.semiconductor nanoparticles such as Si or Ge nanoparticles, can bepre-treated before growth of CNTs is started. An example of such apre-treatment may be removal of a native oxide (e.g. SiO₂ in case of Sinanoparticles) by means of, for example a HF dip (e.g. a dip in 2% HFfor 5 minutes).

After formation of the non-metal containing nanoparticles 14, thesubstrate 10 on which the nanoparticles 14 are formed, or according toalternative embodiments the bulk nanoparticles, are transferred to asuitable reactor chamber of a reactor such as a Chemical VaporDeposition (CVD) reactor to grow the CNTs 16 (see step 15 in FIG. 1).The CVD reactor can, for example, be a Plasma Enhanced CVD reactor or aThermal CVD reactor. In the CVD reactor a carbon source gas isdecomposed or cracked by heating it. Cracking the carbon source gasleads to formation of different carbon fragments such that thesefragments can be recombined on the catalyst nanoparticles to form a CNT.Recombination thus takes place at a surface of the formed nanoparticles14, e.g. semiconductor comprising nanoparticles such as Si or Gecomprising nanoparticles.

Heating the carbon source gas may, according to embodiments of theinvention, be done by using a hot filament, by using a plasma or byusing a combination of a hot filament and a plasma. When using a hotfilament for decomposing the carbon source gas, this hot filament may belocated in the reactor chamber such that cracked or decomposed carbonspecies do not recombine before they have reached the catalystnanoparticles so as to grow CNTs (see further). The hot filament may bea metallic filament and can comprise W (Tungsten) or Ta (Tantalum) andis kept at high temperatures. The height of the temperature depends onthe carbon source used and needs to be high enough to crack the carbonsource. For example, the temperature of the hot filament may be 950° C.or higher.

During formation of reactive carbon fragments by decomposing the carbonsource gas and during subsequent CNT growth, the temperature of thecatalyst nanoparticles 14 and/or the substrate 10 on which the catalystnanoparticles 14 are formed may be in the range of between 800° C. up to1000° C.

According to embodiments of the invention, any suitable carbon sourcegas known by a person skilled in the art may be used. For example thecarbon source gas may be a hydrocarbon source and may be a hydrocarbongas having one (C1) up to three (C3) carbon atoms. Examples of suitablehydrocarbon gases to be used for CVD assisted CNT growth may be CH₄,C₂H₄, C₂H₂ or C₃H₆. According to other embodiments, alternative carbonsources such as carbon oxide (CO) can also be used as a carbon sourcegas. The amount of carbon source gas used in the reactor chamberdetermines the growth, morphology and properties of CNTs formed. Theamount of carbon gas and/or the amount of cracked carbon fragments inthe reactor chamber should be sufficient, i.e. high enough, to achieveCNT growth but on the other hand should be low enough so as to avoidformation of amorphous carbon onto the catalyst nanoparticles as in thatcase no CNT growth will occur.

The CVD reactor chamber may furthermore comprise an inert gas andhydrogen. The inert gas may, for example, be nitrogen. As an example thetotal flow of gasses in the CVD reactor during the step of forming CNTs16 may be around 4 l/min N₂, 2 l/min H₂ and 0.01 up to 1 l/min carbongas such as e.g. C₂H₂. A suitable gas flow can be 4 l/min N₂, 2 l/min H₂and 0.1 l/min carbon gas such as C₂H₂.

Examples of a simplified reactor which may be used to perform CNT growthaccording to embodiments of the present invention is schematicallyillustrated in FIG. 2 and FIG. 3. The difference between FIG. 2 and FIG.3 is the location of the hot filament 2. The reactor comprises a quartztube 6 in which the substrate 10 comprising the nanoparticles 14, e.g.semiconductor comprising nanoparticles such as Si or Ge comprisingnanoparticles, is placed. A furnace 3 is situated at the outside of thequartz tube 6 and is used to create an optimal reaction temperaturewithin the quartz tube 6. With optimal temperature is meant atemperature at which CNT growth can take place. In the example given inFIG. 2, a hot filament 2 is placed at the entrance or gas inlet 1 of thereactor such that the carbon source, e.g. carbon source gas is crackedinto fragments which may then recombine on the nanoparticles 14 to formCNTs 16. According to other embodiments and as illustrated in FIG. 3,the hot filament 2 may be located above the substrate 10. In the lattercase, more extensive growth of CNTs may be obtained because in that casethe cracked carbon species do not have to travel a long way to reach thecatalyst nanoparticles 14, and thus have a lower chance, with respect tothe case illustrated in FIG. 2, of recombining before having assisted inCNT growth.

To release the CNTs 16 formed on the substrate 10, for, for example,bulk production of CNTs 16, a simple release process such as, forexample, chemical dissolution of the substrate 10 can be done.

Hereinafter, some examples will be described. It has to be understoodthat these are only for the ease of understanding the present inventionand are not intended to limit the invention in any way.

Examples

1. Nanoparticles Preparation

A silicon wafer was provided as a substrate 10 to grow the CNTs 16 on.Onto the silicon substrate 10, first a Si₃N₄ barrier layer 11 wasdeposited in a vacuum reactor. Onto the Si₃N₄ barrier layer 11 a thinlayer 12 of 5 nm poly-Si was deposited. Without breaking the vacuum thesample was annealed in conditions such that the thin layer 12 broke intonanoparticles 14. The anneal step to break up the poly-Si layer 12 intoSi nanoparticles 14 was performed at 530° C. during a time period of 20minutes. The obtained Si nanoparticles 14 had a diameter ofapproximately 5 nm.

2. Catalyst Nanoparticle Pre-treatment

The substrate 10 with the nanoparticles 14 on was then placed in astandard HF solution (2% HF) for a couple of minutes (e.g. 5 minutes) atroom temperature so as to remove a possibly present native oxide formedafter the nanoparticles have been exposed to air. Immediately afterremoval of the native oxide, the substrate 10 comprising the Sinanoparticles 14 was placed in a CVD reactor at 900° C. for 5 min. Thereactor gases were N₂ and H₂ at a ratio of 4 l/min N₂ to 4 l/min H₂. TheSi nanoparticles 14 were found to be suitable for growing CNTs 16according to a method of embodiments of the present invention. By thenanoparticles 14 being suitable is meant that they can act as a templateor precursor for CNT formation, in other words, that they can be used toinitiate CNT growth.

3. CNT Growth

After formation of the Si catalyst nanoparticles 14 in the CVD reactor,C₂H₂ gas was added to the reactor at a flow of 0.5 l/min. N₂ and H₂ werealso present in the reactor chamber during CNT growth at a ratio of 4l/min N₂ to 2 l/min H₂. The substrate temperature was in the range ofbetween 800° C. and 1000° C., for example 900° C.

During CNT growth a W or Ta filament 2, situated at the entrance 1 ofthe gas inlet of the reactor, was heated such that incoming C₂H₂ gas wascracked into different carbon fragments such as C—C, C—H, CH₃• radicals,as well as stable species like CH₄ or C₂H₂. The filament temperature wasaround 950° C. (filament current was 6½-6¾ A). FIG. 4 illustrates ascanning electron microscopy (SEM) picture of CNTs grown onto Sicatalyst nanoparticles. It can be seen from FIG. 4 that, in theexperiment conducted, nanotubes are grown at a pm scale distance.

4. Massive CNT Growth

Before performing CNT growth, the substrates 10 with nanoparticles 14 onwere etched in HF (2%) for 1 min. at room temperature in order to removea possibly present native oxide from the Si nanoparticles 14.

The samples were placed in the CVD reactor chamber with temperaturesranging between 600° C. and 9000° C., under reducing atmosphere in N₂:H₂(4:2 l/min.) for 5 minutes at atmospheric pressure.

A W wire was used as a hot filament 2 and was located above thesubstrate 10 comprising the catalyst nanoparticles 14. A flow of 0.1l/min. of acetylene, ethylene or methane in addition to the other gases(N₂:H₂) was flown over the hot filament 2 such that the Carbon sourcegas was cracked. The gas composition used for this experiment wasN₂:H₂:C at a ratio 4:2:0.1 l/min. The Carbon source used was either oneof acetylene, ethylene or methane. The CNTs 16 were grown for half anhour at atmospheric pressure.

FIG. 5 and FIG. 6 illustrate SEM pictures after growth of CNTs onto theSi nanoparticles according to embodiments of the present example.Massive growth of CNTs 16 was observed, i.e. CNTs were grown much closerto each other when compared to FIG. 4. A more intensive growth wasobserved in the area where the Si nanoparticles are closer to the hotfilament 2 (see upper row of CNTs 16 in FIG. 5).

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope of this invention as defined by the appendedclaims.

1-18. (canceled)
 19. A method for forming at least one carbon nanotube,the method comprising: providing at least one metal-free catalystnanoparticle; and growing a carbon nanotube from the metal-free catalystnanoparticle.
 20. The method of claim 19, wherein providing at least onemetal-free catalyst nanoparticle comprises providing at least onemetal-free catalyst nanoparticle in a chemical vapor deposition reactor,and wherein growing a carbon nanotube comprises forming reactive carbonfragments by decomposing a carbon source gas in a chemical vapordeposition reactor and recombining the reactive carbon fragments on topof the at least one metal-free catalyst nanoparticle, whereby at leastone carbon nanotube is grown.
 21. The method of claim 20, wherein thecarbon source gas is a hydrocarbon gas having from one to three carbonatoms.
 22. The method of claim 21, wherein the carbon source gas isselected from the group consisting of CH₄, C₂H₄, C₂H₂, and C₃H₆.
 23. Themethod of claim 20, wherein the carbon source gas is CO.
 24. The methodof claim 20, wherein providing at least one metal-free catalystnanoparticle in a chemical vapor deposition reactor comprises providingat least one metal-free catalyst nanoparticle on a substrate andtransferring the substrate with the at least one metal-free nanoparticleon it to a chemical vapor deposition reactor.
 25. The method of claim24, wherein providing at least one metal-free catalyst nanoparticle on asubstrate comprises providing a layer of metal-free material on thesubstrate and annealing the layer, whereby at least one metal-freecatalyst nanoparticle is formed.
 26. The method of claim 25, whereinannealing is performed at a temperature of from 500° C. to 800° C. 27.The method of claim 24, wherein the method further comprises, beforeproviding the at least one metal-free catalyst nanoparticle on thesubstrate, providing a barrier layer on the substrate, wherein thebarrier layer prevents interaction of the at least one metal-freecatalyst nanoparticle with the substrate.
 28. The method of claim 24,wherein, during decomposing the carbon source gas and growing the atleast one carbon nanotube, a temperature of the substrate is maintainedat from 800° C. to 1000° C.
 29. The method of claim 20, whereindecomposing the carbon source gas is performed by using a hot filament,by using a plasma, or by using a combination of a hot filament and aplasma.
 30. The method of claim 29, wherein decomposing the carbonsource gas is performed by using a hot filament at a temperature of 950°C.
 31. The method of claim 19, wherein the metal-free catalystnanoparticle is a semiconductor comprising nanoparticle.
 32. The methodof claim 31, wherein the semiconductor comprising nanoparticle isselected from the group consisting of a SiC comprising nanoparticle, aSiO₂ comprising nanoparticle, a pure silicon nanoparticle, a GeO₂comprising nanoparticle, and a pure Ge nanoparticle.
 33. The method ofclaim 19, wherein the at least one metal-free catalyst nanoparticle hasa diameter of from 0.4 nm to 100 nm.
 34. The method of claim 19, furthercomprising pre-treating the at least one metal-free catalystnanoparticle before growing the carbon nanotube.
 35. A carbon nanotubegrown from a metal-free catalyst nanoparticle.
 36. Use of a metal-freecatalyst nanoparticle to grow a carbon nanotube.